The bones and connective tissue of an adult human spinal column include more than twenty vertebrae coupled sequentially to one another by a tri-joint complex. The complex includes an anterior disc and two posterior facet joints. The anterior discs of adjacent bones are cushioned by cartilage spacers referred to as intervertebral discs. The vertebrae are each anatomically categorized into one of four classifications: cervical, thoracic, lumbar, and sacral. The cervical portion of the spine, which comprises the top of the spine up to the base of the skull, includes the first seven vertebrae. The intermediate twelve vertebrae are thoracic vertebrae, and connect to the lower spine comprising five lumbar vertebrae. The base of the spine includes the sacral bones (including the coccyx).
The spinal column is highly complex in that it includes over twenty vertebrae coupled to one another for housing and protecting critical elements of the nervous system. These elements of the nervous system include numerous peripheral nerves and circulatory bodies in close proximity to each other. Despite its complexity, the spine is a highly flexible structure, capable of a high degree of curvature and twisting in many different directions.
Spinal pathologies can arise that either limit the range of motion, or threaten the critical elements of the nervous system protected by the spinal column. These pathologies can arise from genetic or developmental irregularities, trauma, chronic stress, tumors and/or disease. A variety of systems are known in the art that provide some degree of immobilization of the spine by implanting artificial assemblies in or onto the spinal column. These assemblies include anterior, posterior, and lateral assemblies. Lateral and anterior assemblies can be coupled to the anterior portion of the spine, typically between vertebral bodies. Posterior spinal fixation systems generally include a pair of rods, which can be aligned along an axis to which the bones are to be disposed, and which are then attached to the spinal column by spinal fixation bone anchors, such as pedicle hooks and/or pedicle screws. Hooks can be coupled to the lamina or attached to transverse processes, while screws can be inserted through pedicles. In order to provide enhanced torsional rigidity, these structures can include cross-connecting devices for coupling the rods together in a direction that is generally transverse with respect to the axis of the rods. These cross-connecting devices can be coupled directly to the rods themselves, or can be attached to the bone anchors. Spinal fixation devices may be surgically implanted in the body to effect a desired relationship between adjacent vertebral bodies. Such devices typically include a rigid stabilizing rod coupled to one or more devices for anchoring the rod to the vertebral bodies. The stabilizing rod must be contoured to accommodate variations in patient anatomy and/or accomplish the desired therapeutic benefits. Since each vertebral body varies in size and shape, a variety of anchoring devices have been developed, including pedicle screws. Pedicle screws have a shape and size appropriate for engaging pedicle bone and may be used to attach external hardware such as rods and plates to the vertebrae.
Traditional pedicle screws are rigid bone screws that, after installation, absorb significant loading in the vertebrate body. Compared with loading the bone itself, loading of the screw weakens the vertebrate bone structure in the vertebrate body and hinders proper load transfer over time due to stress shielding. In some extreme cases, the bone screw may even fracture within the bone, causing pain and discomfort for the patient while also reducing the overall strength of the spinal fixation device.
Although it is undesirable, failure can occur in pedicle screws and such may jeopardize spinal alignment and fixation stability and may lead to severe complications. Loosening of a pedicle screw can occur in a small minority of patients, particularly in osteoporotic spines. It is believed that screw loosening is related to the low pull-off strength of a screw, which measures a screw's purchase power in bone. A screw with a higher pull-off strength generally has a smaller chance of loosening, and thus lead to higher success rate of surgery. In addition, most pedicle screws are rigid and have a very high structural or bending stiffness compared to the surrounding bone. Due to its high structural stiffness, a rigid metal pedicle screw will absorb most of the external loads and unloads the vertebrate bone, a phenomenon called “stress shielding.” According to Wolf's law, a bone with less loading will eventually become weaker. This unnatural loading pattern may cause further degradation of the spinal column.
Accordingly, a bone screw that allows for increased loading on the bone in order to promote bone fusion and prevent adjacent level degradation is desired. In addition, a bone screw that allows for increased osteointegration through automatic and self-managed expansion after insertion into the bone is desired.